The present invention relates to a process for producing alkali pulp by cooking chip-form cellulosic raw materials using an alkaline cooking liquor and subsequently oxygen-alkali delignifying the cooked cellulosic materials with an alkaline aqueous medium containing dissolved oxygen therein, in a single pressurized reaction vessel.
There has already been proposed a process for alkali cooking chip-form cellulosic raw materials, washing the cooked cellulosic materials, and subjecting them to liquor-displacement delignification using an alkaline aqueous medium containing dissolved oxygen therein in a single pressurized reaction vessel (see Australian Pat. No. 495,331).
The above-described process employs a single, cylindrical pressurized reaction vessel in which an alkali cooking zone, a washing zone, and a liquor-displacement delignifying zone are provided in that order. Chip-form cellulosic raw materials are introduced into the reaction vessel, and are subjected to alkali cooking and washing, and are transferred within the reaction vessel from the washing zone to the delignifying zone. In the liquor-displacement delignifying zone, the cooked cellulosic materials, still in chip-form, are contacted with an alkaline aqueous medium containing dissolved oxygen to proceed with oxidative delignification. A part of waste liquor, formed during delignification, is continuously discharged from the delignifying zone, while fresh alkaline aqueous medium containing dissolved oxygen is continuously charged into the delignifying zone, whereby the continuous displacement of the waste liquor with the fresh alkaline aqueous medium containing dissolved oxygen is carried out. The thus delignified cellulosic materials are removed from the reaction vessel.
In order to introduce the alkaline aqueous medium containing dissolved oxygen into the delignifying zone in the above-described process, an operation as shown in FIG. 1 has been conducted. Namely, in FIG. 1, the cooked cellulosic materials descend inside the pressurized reaction vessel 1 from the washing zone 2 to the delignifying zone 3. The alkaline aqueous medium containing dissolved oxygen is sprayed through a central feeding pipe 4 onto the descending cellulosic materials. If desired, the alkaline aqueous medium may also be introduced into the delignifying zone from a nozzle 5 provided at the side wall of the vessel 1. The waste liquor formed during delignification is continuously discharged through a strainer 6 provided at the peripheral portion of the vessel. The cooked and oxygen-alkali delignified cellulosic materials then reach a diluting zone 7 where they are diluted with a washing filtrate sent from a washer 8, and are discharged from a discharging outlet 9, and then are sent to the washer 8 where they are washed with water.
The above-mentioned operation for introducing alkaline aqueous medium containing dissolved oxygen, however, is not necessarily satisfactory from the standpoint of uniform contact of cellulosic materials with the alkaline aqueous medium, and homogeneous delignification.
There is another prior art process for producing alkali pulp using a single, cylindrical pressurized reaction vessel in which a liquor-impregnating zone, a cooking zone, a washing zone and a diluting zone are provided in that order. In the end portion of the diluting zone, a motor-driven scraping or agitating device and a pulp discharging outlet are provided. Chip-form cellulosic raw materials are subjected to impregnation of alkali cooking liquor, alkali cooking, washing and diluting, in succession, during their downward movement inside the reaction vessel. Thus cooked cellulosic materials are finally discharged from the discharging outlet (see, for example, U.S. Pat. No. 3,298,899).
However, there is no disclosure of a process for producing alkali pulp by alkali cooking, washing and oxygen-alkali delignifying cellulosic materials successively in such conventional cylindrical single pressurized reaction vessel.